add cmp.get_registered_sources and CmpRegisterSource /CmpUnregisterSource autocmd.

lua/cmp/core.lua

@@ -47,8 +47,11 @@ end
 
 ---Unregister source
 ---@param source_id integer
+---@return cmp.Source?
 core.unregister_source = function(self, source_id)
+  local s = self.sources[source_id]
   self.sources[source_id] = nil
+  return s
 end
 
 ---Get new context
@@ -105,6 +108,12 @@ core.get_sources = function(self, filter)
   return sources
 end
 
+---Return registered sources.
+---@return cmp.Source[]
+core.get_registered_sources = function(self)
+  return self.sources
+end
+
 ---Keypress handler
 core.on_keymap = function(self, keys, fallback)
   local mode = api.get_mode()